Background Relating to Optical Amplifiers
Single-mode rare-earth-doped optical fiber amplifiers have been widely used for over a decade to provide diffraction-limited optical amplification of optical pulses. Because single mode fiber amplifiers generate very low noise levels, do not induce modal dispersion, and are compatible with single mode fiber optic transmission lines, they have been used almost exclusively in telecommunication applications.
The amplification of high peak-power pulses in a diffraction-limited optical beam in single-mode optical fiber amplifiers is generally limited by the small fiber core size that needs to be employed to ensure single-mode operation of the fiber. In general the onset of nonlinearities such as self-phase modulation lead to severe pulse distortions once the integral of the power level present inside the fiber with the propagation length exceeds a certain limiting value. For a constant peak power P inside the fiber, the tolerable amount of self-phase modulation Φnl is given by
            Φ      nl        =                            2          ⁢          π          ⁢                                          ⁢                      n            2                    ⁢          PL                          λ          ⁢                                          ⁢          A                    ≤      5        ,where A is the area of the fundamental mode in the fiber, ë is the operation wavelength, L is the fiber length and n2=3.2×10−29 m2/W is the nonlinear refractive index in silica optical fibers.
As an alternative to single-mode amplifiers, amplification in multi-mode optical fibers has been considered. However, in general, amplification experiments in multi-mode optical fibers have led to non-diffraction-limited outputs as well as unacceptable pulse broadening due to modal dispersion, since the launch conditions into the multi-mode optical fiber and mode-coupling in the multi-mode fiber have not been controlled.
Amplified spontaneous emission in a multi-mode fiber has been reduced by selectively exciting active ions close to the center of the fiber core or by confining the active ions to the center of the fiber core. U.S. Pat. No. 5,187,759, hereby incorporated herein by reference. Since the overlap of the low-order modes in a multi-mode optical fiber is highest with the active ions close to the center of the fiber core, any amplified spontaneous emission will then also be predominantly generated in low-order modes of the multi-mode fiber. As a result, the total amount of amplified spontaneous emission can be reduced in the multi-mode fiber, since no amplified spontaneous emission is generated in high-order modes.
As an alternative for obtaining high-power pulses, chirped pulse amplification with chirped fiber Bragg gratings has been employed. One of the limitations of this technique is the relative complexity of the set-up.
More recently, the amplification of pulses to peak powers higher than 10 KW has been achieved in multi-mode fiber amplifiers. See U.S. Pat. No. 5,818,630, entitled Single-Mode Amplifiers and Compressors Based on Multi-Mode Fibers, assigned to the assignee of the present invention, and hereby incorporated herein by reference. As described therein, the peak power limit inherent in single-mode optical fiber amplifiers is avoided by employing the increased area occupied by the fundamental mode within multi-mode fibers. This increased area permits an increase in the energy storage potential of the optical fiber amplifier, allowing higher pulse energies before the onset of undesirable nonlinearities and gain saturation. To accomplish this, that application describes the advantages of concentration of the gain medium in the center of the multi-mode fiber so that the fundamental mode is preferentially amplified. This gain-confinement is utilized to stabilize the fundamental mode in a fiber with a large cross section by gain guiding.
Field of the Invention
The present invention relates to the use of multi-mode fibers for amplification of laser light in a single-mode amplifier system.
Description of the Related Art
Rare-earth-doped optical fibers have long been considered for use as sources of coherent light, as evidenced by U.S. Pat. No. 3,808,549 to Maurer (1974), since their light-guiding properties allow the construction of uniquely simple lasers. However, early work on fiber lasers did not attract considerable attention, because no methods of generating diffraction-limited coherent light were known. Man current applications of lasers benefit greatly from the presence of diffract on limited light.
Only when it became possible to manufacture single-mode (SM) rare-earth-doped fibers, as reported by Poole et al. in “Fabrication of Low-Loss Optical Fibres Containing Rare-Ear Ions”, Optics Letters, Vol. 22, pp. 737-738 (1985), did the rare-earth-doped optical fiber technology become viable. In this technique, only the fundamental mode of the optical fiber is guided at the lasing wavelength, thus ensuring diffraction-limited output.
Driven by the needs of optical fiber telecommunications for SM optical fiber amplifiers, nearly all further developments for more than a decade in this area were concentrated on perfecting SM fiber amplifiers. In particular, the motivation for developing SM fiber amplifiers stemmed from the fact that SM fiber amplifiers generate the least amount of noise and they are directly compatible with SM fiber optic transmission lines. SM fiber amplifiers also have the highest optical transmission bandwidths, since, due to the absence of any higher-order modes, modal dispersion is completely eliminated. In general, modal dispersion is the most detrimental effect limiting the transmission bandwidth of multi-mode (MM) optical fibers, since the higher-order modes, in general, have different propagation constants.
However, in the amplification of short-optical pulses, the use of SM optical fibers is disadvantageous, cause the limited core area limits the saturation energy of the optical fiber and thus the obtainable pulse energy. The saturation energy of a laser amplifier can be expressed as
            E      sat        =                  h        ⁢                                  ⁢        υ        ⁢                                  ⁢        A            σ        ,where h is Planck's constant, υ is the optical frequency, a is the stimulated emission cross section and A is the core area. The highest pulse energy generated from a SM optical fiber to date is about 160 μJ (disclosed by Taverner et al. in Optics Letters, Vol. 22, pp. 378-380 (1997), and was obtained from a SM erbium-doped fiber with a core diameter of 15 μm, which is about the largest core diameter that is compatible with SM propagation at 1.55 μm. This result was obtained with a fiber numerical aperture of NA≈0.07. Any further increase in core diameter requires a further lowering of the NA of the fiber and results in an unacceptably high sensitivity to bend-losses.
As an alternative to SM amplifiers amplification in multi-mode (MM) optical fibers has been considered. See, for example, “Chirped-pulse amplification of ultrashort pulses with a multi-mode Tm:ZBLAN fiber upconversion amplifier” by Yang et al., Optics Letters, Vol. 20, pp. 1044-1046 (1995). However, in general, amplification experiments in MM optical fibers have led to non-diffraction-limited outputs as well as unacceptable pulse broadening due to modal dispersion, since the launch conditions into the MM optical fiber and mode-coupling in the MM fiber were not controlled.
It was recently suggested by Griebner et al. in “‘Efficient laser operation with nearly diffraction-limited output from a diode-pumped heavily Nd-doped multi-mode fiber”, Optics Letters, Vol. 21, pp. 266-268 (1996), that a near diffraction-limited output be can be obtained from a MM fiber laser when keeping the fiber length shorter than 15 mm and selectively providing a maximum amount of feedback for the fundamental mode of the optical fiber. In this technique, however, severe mode-coupling was a problem, as the employed MM fibers supported some 10,000 modes. Also, only an air-gap between the endface of the MM fiber and a laser mirror was suggested for mode-selection. Hence, only very poor modal discrimination was obtained, resulting in poor beam quality.
In U.S. Pat. No. 5,187,759 to DiGiovanni et al., it was suggested that amplified spontaneous emission (ASE) in a MM fiber can be reduced by selectively exciting any active ions lose to the center of the fiber core or by confining the active ions to the center of the fiber core. Since the overlap of the low-order modes in a MM optical fiber is highest with the active ions close to the center of the fiber core, any ASE will then also be predominantly generated in low-order modes of the MM fiber. As a result, the total amount of ASE can be greatly reduced in MM fiber, since no ASE is generated in high-order modes. However, DiGiovanni described dopant confinement only with respect to ASE reduction. DiGiovanni did not suggest that, in the presence of mode-scattering, dopant confinement can enhance the beam quality of the fundamental mode of the M fiber under SM excitation. Also, the system of DiGiovanni did not take into account the fact that gain-guiding induced by dopant confinement can in fact effectively guide a fundamental mode in a MM fiber. This further reduces ASE in MM fibers as well as allowing for SM operation.
In fact, the system of DiGiovanni et al. is not very practical, since it considers a MM signal source, which leads to a non-diffraction-limited output beam. Further, only a single cladding was considered for the doped fiber, which is disadvantageous when trying to couple high-power semi-conductor lasers into the optical fibers. To couple high-power semiconductor lasers into MM fibers, a double-clad structure, as suggested in the above-mentioned patent to Maurer, can be of an advantage.
To the inventors' knowledge, gain-guiding has not previously been employed in optical fibers. On the other hand, gain-guiding is well known in conventional semiconductor and solid-state lasers. See, for example, “‘Alexandrite-laser-pumped Cr3+:Li rA1F6” by Harter et al., Optics Letters, Vol. 17, pp. 1512-1514 (1992). Indeed, in SM fibers, gain-guiding is irrelevant due to the strong confinement of the fundamental mode by the wave-guide structure. However, in MM optical fibers, the confinement of the fundamental mode by the waveguide structure becomes comparatively weaker, allowing for gain-guiding to set in. As the core size in a MM fiber becomes larger, light propagation in the fiber structure tends to approximate free-space propagation. Thus, gain-guiding can be expected eventually to be significant, provided mode-coupling can be mad sufficiently small. In addition to providing high pulse energies, MM optical fiber amplifiers can also be used to amplify very high peak power pulses due to their increased fiber cross section compared to SM fiber amplifiers. MM undoped fibers and MM amplifier fibers can also be used for pulse compression as recently disclosed by Fermann et al. in U.S. application Ser. No. 08/789,995 (filed Jan. 28, 1997). However, this work was limited to the use of MM fibers as soliton Raman compressors in conjunction with a nonlinear spectral filtering action to clean-up the spectral profile, which may limit the overall efficiency of the system.
Compared to pulse compression in SM fibers, such as that disclosed in U.S. Pat. No. 4,913,520 to Kafka et al., higher-pulse energies can be obtained in MM fibers due to the increased mode-size of the fiber. In particular, V-values higher than 2.5 and relatively high index differences between core and cladding (i.e. a Δn>0.3%) can be effectively employed. In “Generation of high-energy 10-fs pulses by a new pulse compression technique”, Conference on Lasers and Electro-Optics, CLEO 91, paper DTuR5, Optical Society of America Technical Digest Series, #9, pp. 189-190 (1996), M. Nisoli et al. suggested the use of hollow-core fibers for pulse-compression, as hollow-core fibers allow an increase in the mode size of the fundamental mode. However, hollow-core fibers have an intrinsic transmission loss, they need to be filled with gas, and they need to be kept straight in order to minimize the transmission losses, which makes them highly impractical.
As an alternative to obtaining high-power pulses, chirped pulse amplification with chirped fiber Bragg gratings may be employed, as disclosed in U.S. Pat. No. 5,499,134 to Galvanauskas et al. (1996). One of the limitations of this technique is that, in the compression grating, a SM fiber with a limited core area is employed. Higher pulse energies could be obtained by employing chirped fiber Bragg gratings in MM fibers with reduced mode-coupling for pulse compression. Indeed, unchirped fiber Bragg gratings were recently demonstrated in double-mode fibers by Strasser et al. in “Reflective-mode conversion with UV-induced phase gratings in two-mode fiber”, Optical Society of America Conference on Optical Fiber Communication, OFC97, pp. 348-349, (1997). However, these gratings were blazed to allow their use as mode-converters, i.e., to couple the fundamental mode to a higher-order mode. The use of Bragg gratings in pulse-compression calls for an unblazed grating to minimize the excitation of any higher-order modes in reflection.
It has long been known that a SM signal can be coupled into a MM fiber structure and preserved for propagation lengths of 100s of meters. See, for example, “Pulse Dispersion for Single-Mode Operation of Multi-mode Cladded Optical Fibres”, Gambling et al., Electron. Lett., Vol. 10, pp. 148-149, (1974) and “Mode conversion coefficients in optical fibers”, Gambling et al., Applied Optics, Vol. 14, pp. 1538-1542, (1975). However, Gambling et al. found low levels of mode-coupling only in liquid-core fibers. On the other hand, mode-coupling in MM solid-core fibers was found to be severe, allowing for the propagation of a fundamental mode only in mm lengths of fiber. Indeed, as with the work by Griebner et al., Gambling et al. used MM solid-core optical fibers that supported around 10,000 or more modes.
In related work, Gloge disclosed in “Optical Power Flow in Multi-mode Fibers”, The Bell System Technical Journal, Vol. 51, pp. 1767-1783, (1972), the use of MM fibers that supported only 700 modes, where mode-coupling was sufficiently reduced to allow SM propagation over fiber lengths of 10 cm.
However, it was not shown by Gloge that mode-coupling can be reduced by operating MM fibers at long wavelengths (1.55 μm) and by reducing the total number of modes to less than 700. Also, in this work, the use of MM fibers as amplifiers and the use of the nonlinear properties of MM fibers was not considered.
The inventors are not aware of any prior art using MM fibers to amplify SM signals where the output remains primarily in the fundamental mode, the primary reason being that amplification in MM fibers is typically not suitable for long-distance signal propagation as employed in the optical telecommunication area. The inventors arc also not aware of any prior art related to pulse compression in multi-mode fibers, where the output remains in the fundamental mode.
All of the above-mentioned articles, patents and patent applications are hereby incorporated herein by reference.